Semiconductor elements, such as transistors and IC chips, are encapsulated into ceramic or plastic packages to be supplied as semiconductor devices protected from the outer environment and easy to handle. Ceramic packages have excellent moisture resistance because of the character of the ceramic material itself and impose little stress to the semiconductor element because of their hollow structure. Ceramic packages therefore achieve highly reliable encapsulation. However, the ceramic materials are expensive, and the ceramic packages are less practical for mass production than plastic packages.
Therefore, plastic packages using an epoxy resin composition have been leading recently. Plastic packages, while suitable for mass production and less expensive, allow moisture to permeate and have a greater linear expansion coefficient as compared with a semiconductor element encapsulated. Therefore, it has been a weighty subject in the art to improve moisture resistance and low stress properties.
The stress problem of encapsulating resins has been coped with by dispersing rubber particles, such as butadiene rubber particles, in the resin matrix. However, since rubber particles exhibit high cohesion, commercially available butadiene rubber particles usually have an average secondary particle diameter of about 100 to 500 μm. Such rubber particles fail to be dispersed uniformly in the encapsulating resin, which has imposed a limitation on the improvement in low stress properties required of a semiconductor encapsulation material.